In general, polyimide resins are heat-resistant resins by the dehydrating cyclization of a polyamic acid which is produced by the condensation between an aromatic tetracarboxylic anhydride and an aromatic diamine. The polyimide resins have been extensively used as films, coating materials, molded parts and insulating materials in various applications such as electric, electronic, automobile and aerospace industries because of their exhibit excellent resistance to pyrolysis due to a rigidity of molecular chain, a resonance stability and a strong chemical bond, excellent resistance to chemical changes such as oxidation and hydrolysis, and excellence in mechanical properties, electrical properties and flexibility.
However, in general, films made of the aromatic polyimide resins tend to be colored yellow or brown by the absorption of visible light attributable to the formation of intramolecular or intermolecular electron transfer complexes. Therefore, the polyimide resins are unsuitable for optical applications such as materials of a substrate for flat panel displays and mobile phones, and optical fibers, light wave guides and optical adhesives. In the optical applications, it has been keenly demanded to develop highly heat-resistant, transparent resins which combine a good flexibility, a good resistance to heat discoloration and an excellent mechanical strength.
Polymethyl methacrylates conventionally used as optical plastics have a low birefringence and a colorless transparency, but are lacking in heat resistance, and therefore, unusable in the optical applications. Also, polycarbonates have a relatively high glass transition temperature, but are unsatisfactory in the resistance to heat discoloration which is required in the optical applications and further have a high birefringence, thereby failing to meet the requirements for the optical applications.
Sealing materials for photoelectric transfer devices such as light-emitting diodes (LED) and optical sensors have been required to have a colorless transparency, an easy moldability and a heat resistance. In particular, as the sealing materials for LED, there have been extensively used epoxy resins that are excellent in these properties (Patent Document 1). With the recent progress of development of light-emitting materials, LED capable of not only exhibiting a higher luminance but also emitting light having a shorter wavelength such as blue light and ultraviolet light tends to become predominant. With such a tendency, the sealing materials used for LED are frequently exposed to a high temperature and a high-energy light radiation. Known epoxy resins are lacking in heat resistance and light resistance, therefore, tend to suffer from discoloration (yellowing), etc. Thus, the epoxy resins have failed to maintain a colorless transparency for a long period of time. From the standpoint of environmental protection, lead-free soldering is prevailing, which causes such a tendency that the temperature used upon mounting the LED also increases. Therefore, the lead-free soldering is hardly applicable to known epoxy resins having a poor heat resistance.
To solve the above problems, it has been attempted to use silicone resins having excellent heat resistance and light resistance as the sealing material. However, the silicone resins have defects such as poor adhesion and brittleness. Therefore, devices using the silicone resins tend to be unsatisfactory in reliability. Further, when using the silicone-based resins as the sealing material, an electric defective contact tends to occur owing to volatile components generated from the resins under a high-temperature condition for mounting the devices. Further, according to the Snell's law, in order to enhance a light extraction efficiency of LED, the sealing material preferably exhibits a higher refractive index in the wavelength range of light emitted therefrom. However, known epoxy resins and silicone resins generally have a refractive index of less than 1.55. Thus, it has been demanded to develop sealing materials having a higher refractive index.
Further, the epoxy resins or silicone-based resins conventionally used for sealing LED are thermosetting resins requiring a long curing time upon the sealing, resulting in problems such as a prolonged time for production of LED. To solve these problems, there has been proposed a method of injection-molding a thermoplastic resin for sealing LED (Patent Document 2). However, in this method, there tends to arise such a problem that the sealing resin is softened and deformed under a high-temperature condition used upon mounting LED, resulting in incomplete sealing of LED.
As materials that are free from these problems, there have been proposed colorless transparent polyimide resins. As the polyimide resins having a high heat resistance and a high transparency, there are described fluorinated polyimide resins having a repeating unit containing a perfluoroalkyl group (Patent Documents 3 and 4). The fluorinated polyimide resins have a poor solubility in a solvent. For this reason, a polyamic acid having a poor storage stability is cast to form a coating film, and then the obtained coating film is heated at a temperature as high as 350° C. for the imidation of the polyamic acid, thereby obtaining a desired film. However, the film is easily discolored yellow due to fluorine when subjected to heat treatments for the film formation and imidation, and suffers from unstable surface smoothness and difficulty in controlling the thickness.
There is disclosed a method of producing a heat-fusible polyimide resin by using 1,2,4,5-cyclohexanetetracarboxylic dianhydride (Patent Document 5). In Examples thereof, 1,2,4,5-cyclohexanetetracarboxylic dianhydride is reacted with diaminodiphenylmethane to obtain a polyamic acid which is then imidated under heating, and the polyimide obtained is subjected to heat-pressure formation to produce a transparent yellow polyimide resin film having a glass transition temperature of 304° C. In addition, it is reported that a less-discolored transparent film having a glass transition temperature of 300° C. or higher is obtained from a polyimide resin solution prepared from 1,2,4,5-cyclohexanetetracarboxylic dianhydride and diaminodiphenyl ether (Patent Document 6). The method described in Patent Document 5 includes the imidation step at a high temperature similarly to the conventional methods, and therefore, fails to exhibit a sufficient effect of preventing the polyimide resin from being undesirably discolored. In addition, the polyimide resin films described in both the patent documents have a high water absorption, and therefore, tends to be deteriorated in dimensional stability upon moisture absorption.    Patent Document 1: JP 2001-158816A    Patent Document 2: JP 4-248864A    Patent Document 3: JP 8-143666A    Patent Document 4: JP 8-225645A    Patent Document 5: U.S. 3,639,343    Patent Document 6: JP 2003-168800A